Domain State Model for Exchange Bias

Monte Carlo simulations of a system consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer described by a classical spin model show a strong dependence of the exchange bias on the degree of dilution in agreement with recent experimental observations on Co/CoO bilayers. These simulations reveal that diluting the antiferromagnet leads to the formation of domains in the volume of the antiferromagnet carrying a remanent surplus magnetization which causes and controls exchange bias. To further support this domain state model for exchange bias we study in the present paper the dependence of the bias field on the thickness of the antiferromagnetic layer. It is shown that the bias field strongly increases with increasing film thickness and eventually goes over a maximum before it levels out for large thicknesses. These findings are in full agreement with experiments.Comment: 8 pages latex, 3 postscript figure

Modeling exchange bias microscopically

Exchange bias is a horizontal shift of the hysteresis loop observed for a ferromagnetic layer in contact with an antiferromagnetic layer. Since exchange bias is related to the spin structure of the antiferromagnet, for its fundamental understanding a detailed knowledge of the physics of the antiferromagnetic layer is inevitable. A model is investigated where domains are formed in the volume of the AFM stabilized by dilution. These domains become frozen during the initial cooling procedure carrying a remanent net magnetization which causes and controls exchange bias. Varying the anisotropy of the antiferromagnet we find a nontrivial dependence of the exchange bias on the anisotropy of the antiferromagnet.Comment: 7 pages, 5 figure

Estimated heats of fusion of fluoride salt mixtures suitable for thermal energy storage applications

The heats of fusion of several fluoride salt mixtures with melting points greater than 973 K were estimated from a coupled analysis of the available thermodynamic data and phase diagrams. Simple binary eutectic systems with and without terminal solid solutions, binary eutectics with congruent melting intermediate phases, and ternary eutectic systems were considered. Several combinations of salts were identified, most notable the eutectics LiF-22CaF2 and NaF-60MgF2 which melt at 1039 and 1273 K respectively which posses relatively high heats of fusion/gm (greater than 0.7 kJ/g). Such systems would seemingly be ideal candidates for the light weight, high energy storage media required by the thermal energy storage unit in advanced solar dynamic power systems envisioned for the future space missions

Nucleation theory and the phase diagram of the magnetization-reversal transition

The phase diagram of the dynamic magnetization-reversal transition in pure Ising systems under a pulsed field competing with the existing order can be explained satisfactorily using the classical nucleation theory. Indications of single-domain and multi-domain nucleation and of the corresponding changes in the nucleation rates are clearly observed. The nature of the second time scale of relaxation, apart from the field driven nucleation time, and the origin of its unusual large values at the phase boundary are explained from the disappearing tendency of kinks on the domain wall surfaces after the withdrawal of the pulse. The possibility of scaling behaviour in the multi-domain regime is identified and compared with the earlier observations.Comment: 10 pages Latex, 4 Postscript figure

Mean field and Monte Carlo studies of the magnetization-reversal transition in the Ising model

Detailed mean field and Monte Carlo studies of the dynamic magnetization-reversal transition in the Ising model in its ordered phase under a competing external magnetic field of finite duration have been presented here. Approximate analytical treatment of the mean field equations of motion shows the existence of diverging length and time scales across this dynamic transition phase boundary. These are also supported by numerical solutions of the complete mean field equations of motion and the Monte Carlo study of the system evolving under Glauber dynamics in both two and three dimensions. Classical nucleation theory predicts different mechanisms of domain growth in two regimes marked by the strength of the external field, and the nature of the Monte Carlo phase boundary can be comprehended satisfactorily using the theory. The order of the transition changes from a continuous to a discontinuous one as one crosses over from coalescence regime (stronger field) to nucleation regime (weaker field). Finite size scaling theory can be applied in the coalescence regime, where the best fit estimates of the critical exponents are obtained for two and three dimensions.Comment: 16 pages latex, 13 ps figures, typos corrected, references adde

Evidence For Advective Flow From Multi-Wavelength Observations Of Nova Muscae

We model the UV/optical spectrum of the black hole binary Nova Muscae as a sum of black body emissions from the outer region of an accretion disk. We show for self-consistency that scattering effects in this region are not important. The black hole mass ($M \approx 6 M_\odot$), the inclination angle ($\mu \approx 0.5$) and the distance to the source ($D \approx 5$ kpc) have been constrained by optical observations during quiescence (Orosz et al. 1996). Using these values we find that the accretion rate during the peak was ${\dot M} \approx 8 \times 10^{19}$ g sec$^{-1}$ and subsequently decayed exponentially. We define a radiative fraction ($f$) to be the ratio of the X-ray energy luminosity to the total gravitational power dissipated for a keplerian accretion disk. We find that $f \approx 0.1$ and remains nearly constant during the Ultra-soft and Soft spectral states. Thus for these states, the inner region of the accretion disk is advection dominated. $f$ probably increased to $\approx 0.5$ during the Hard state and finally decreased to $\approx 0.03$ as the source returned to quiescence.Comment: 5 figures. uses aasms4.sty, accepted by Ap

A Review on the Statistical Methods and Implementation to Homogeneity Assessment of Certified Reference Materials in Relation to Uncertainty

An importance of data analysis, methods for homogeneity test and standard uncertainty evaluation associated in any measurement for exact quantification of certified value of any product is vital to be stressed in the scientific community. Herein, we have collectively summarized the detailed discussion on the basics of statistical parameters such as mean, median, mode, standard deviation, variance, range, normal distribution, and central limit theorem. Various statistical analysis methods such asztest,ttest, Chi-squared test, and ANOVA includingFtest have also been discussed in great detail to test the homogeneity of samples for certification of the reference material. The ISO guide 35 (2006) and Guide to Uncertainty in Measurement (GUM) are primarily considered to describe the basic concept of evaluating the associated uncertainty in the light of GUM modelling approach to avoid the error in the measurement which normally occurs in many scientific reports